1. Field of the Invention
The present invention relates to a method and an apparatus for measuring the degree of evaporation of water vapour passing through the skin, in which method a measurement chamber is placed on the surface of skin. The apparatus comprises a measurement chamber placed on the surface of the skin.
2. Description of the Related Art
The skin consists of three main layers, which are stratum corneum, epidermis and dermis. The water content is low in stratum corneum and high in dermis. The water content increases gradually from the surface of epidermis to deeper parts of epidermis. Water vapour diffuses continuously through the skin, because vapour pressure is higher in the dermis than in the external environment. Besides the external humidity, the amount of water vapour passing through the skin depends on the properties of the surface layer of the skin. The skin's function is to protect the human body against e.g. hazardous microbes and maintain the fluid balance of the body. Measuring the evaporation through the skin is an effective tool for the assessment of skin condition.
The devices for measuring the amount of water vapour passing through the skin are called TEWL meters (TEWL=transepidermal water loss). The well known TEWL meters, Evaporimeter from Servo Med (described in the U.S. Pat. No. 4,066,068) and DermaLab from Cortex Technology, are based on a technology where two humidity sensors are fitted in a chamber which is open at both ends. The probe is placed on the skin so that one open end is against the skin and the other open end is left in free contact with the ambient air. Due to the diffusion of water molecules from the skin, the humidity near the skin surface increases and becomes higher than the humidity of the ambient air. This creates a humidity gradient in the chamber. Two measuring sensors inside the chamber measure this humidity gradient. The humidity flow evaporated through the skin surface can be calculated by diffusion equations from the measured humidity gradient.
The drawback of these devices is that they are prone to disturbances caused by ambient airflows Disturbing ambient airflows can be generated e.g. by opening and closing the door or by some other airflow during the measurement.
The patent application FI-981295 describes a method for determining the diffusion flow of moisture to or from the surface of material. In this method a diffusing chamber is formed on the surface to be measured which is situated at the first end of the chamber, and the vapour pressure is determined or assumed to be known at least at two separate points of the chamber The diffusing chamber has another surface that evaporates or binds water vapour, from or into which the moistness is transferred during the measurement. The measuring time is typically approximately 15 minutes. This method is primarily meant for measuring moistness in constructions, and it is neither applicable nor practical for the measurement of water evaporation through skin.